Low temperature vacuum distillation systems have previously been described in which distillate of a liquid descends through small diameter tubes, trapping gaseous impurities, to thereby assist in pumping such impurities from the system. Vacuum distillation systems of this design thus use the liquid being distilled to assist in creating the vacuum by removing these gaseous impurities. Examples of such systems have been taught generally by U.S. Pat. No. 4,269,664, issued May 26, 1981, by U.S. Pat. No. 4,444,623, issued Apr. 24, 1984, and by U.S. Pat. No. 4,762,593, issued Aug. 9, 1988.
It has been found that it is preferable to use a plurality of small diameter tubes rather than a single conduit of equal cross section for a number of reasons. One is that it is easier for a liquid to entrain bubbles more efficiently by employing smaller tubes.
To further assist in the condensation of the distillate, prior to descending through the bubble tubes, a portion of cooled final purified distillate is recycled to the condensation chamber. However, when the vapor pressure of the contained impurity is higher than the vapor pressure of the desired pure distillate, the impurity tends to be continually absorbed and desorbed from the liquid as it cycles through the vacuum distillation system. Consequently, the impurity evades complete removal.
As noted in U.S. Pat. No. 4,269,664 and U.S. Pat. No. 4,444,623, if the liquid being distilled contains impurities having a vapor pressure near or higher than that of the desired pure distillate, these impurities vaporize, migrate, condense and reach the open final distillate collection vessel just as the desired pure distillate does. As a result, such volatile impurities are not removed by the system.
The volatile impurities are able to boil or vaporize at the system operating pressure along with the desirable distillate. The impurity descends along with the desired distillate through the bubble tubes to the reservoir at the base of the bubble tubes. There it remains in a gaseous state entrained in the desirable distillate until it enters the pump immediately downstream of the bubble tube reservoir and is pumped to the final distillate collection vessel. As the liquid distillate and the gaseous impurity pass through this pump, they are compressed, the pressure on the liquids increases, and the gaseous impurity either liquifies or redissolves into the liquid distillate.
Since only gasses can be removed from the distillate at the final collection vessel, any impurities which have either re-liquified or become dissolved in the distillate cannot be removed. As the liquid mixture of desired condensed distillate and undesired impurity are cycled from the open final distillate collection vessel to reenter the vacuum distillation system, the pressure on the liquid drops and the impurity is once again able to re-vaporize. The gaseous impurity is thus continually absorbed and desorbed by the desirable distillate and is unable to be effectively vacated from the system.
The continual presence of this gaseous impurity prevents the pressure in the distillation system from reaching a desired low value. It also prevents the distillate from being purified of this volatile impurity.